In the majority of applications printing proceeds by pressure contact of an ink-laden printing form with an ink-receiving material which is usually plain paper. The most frequently used impact printing technique is known as lithographic printing based on the selective acceptance of oleophilic ink on a suitable receptor.
In recent times however so-called non-impact printing systems have replaced classical pressure-contact printing to some extent for specific applications. A survey is given e.g. in the book “Principles of Non Impact Printing” by Jerome L. Johnson (1986), Palatino Press, Irvine, Calif. 92715, USA.
Among non-impact printing techniques ink-jet printing has become a popular technique because of its simplicity, convenience and low cost. Especially in those instances where a limited edition of the printed matter is needed, ink-jet printing has become a technology of choice. A recent survey on progress and trends in ink-jet printing technology is given by Hue P. Le in Journal of Imaging Science and Technology Vol. 42 (1), January/February 1998.
In ink-jet printing tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the ink drops image-wise onto the ink-receiver. Printing can be accomplished by moving a print head across the ink-receiver or vice versa. Early patents on ink-jet printers include U.S. Pat. Nos. 3,739,393, 3,805,273 and 3,891,121.
The jetting of the ink droplets can be performed in several different ways. In a first type of process called continuous ink-jet printing, the ink stream jetted from an orifice of the print head is broken up, by applying a pressure wave pattern to this orifice, into ink droplets of uniform size and spacing. When the jet break-up mechanism is controlled, an electric charge can be applied to the droplets selectively and reliably as they form from the continuous ink stream. The charged drops passing through an electric field are deflected into a gutter for recuperation, while the uncharged drops proceed directly onto the ink-receiver to form an image or vice versa.
According to a second process the ink droplets can be created by a “drop on demand” method (DOD). A drop-on-demand device ejects ink droplets only when they are needed for imaging on the ink-receiver, thereby avoiding the complexity of drop charging, deflection hardware, and ink collection. In drop-on-demand ink-jet printing, the ink droplet can be formed by means of a pressure wave created by the mechanical motion of a piezoelectric transducer (so-called “piezo method”), or by means of discrete thermal pulses (so-called “bubble jet” method, or “thermal jet” method).
Ink receiving layers for ink-jet recording media are either non-absorptive or absorptive. In absorptive ink-receiving layers the ink is either absorbed by swelling of the layer due to the specific polymers present in the layer, or is absorbed by capillarity, due to the microporous character of the layer.
It is known that the ink-receiving layers in ink-jet recording elements must meet different stringent requirements:                the ink-receiving layer should have a high ink absorbing capacity, so that the dots will not flow out and will not increase in size more than is necessary to obtain a high optical density;        the ink-receiving layer should have a high ink absorbing speed (short ink drying time) so that the ink-droplets will not feather if touched immediately after application;        the ink dots that are applied to the ink-receiving layer should be substantially round in shape and smooth at their peripheries. The dot diameter must be constant and accurately controlled;        the receiving layer must be readily wetted so that there is no “puddling”, i.e. coalescence of adjacent ink dots, and an previously absorbed ink drop should not show any “bleeding”, i.e. overlap with neighbouring or later placed dots;        transparent ink-jet recording elements must have a low haze-value and exhibit excellent transmittance properties;        after being printed the image must have a good resistance regarding water-fastness, light-fastness, and be stable to extreme conditions of temperature and humidity;        the ink-jet recording material must not show any curl or sticky behavior if stacked before or after being printed;        the ink-jet recording element must be able to move smoothly through different types of printers.        
All these properties are often in a trade-off relationship with one another, as it is difficult to satisfy them all at the same time.
It will be readily understood that the optimal composition of an ink is dependent on the ink-jetting method used and on the nature of the ink-receiver to be printed.
Ink compositions for ink-jet typically include the following ingredients: dyes or pigments, water and/or organic solvents, humectants such as glycols, detergents, thickeners, polymeric binders, preservatives, etc.
Ink compositions can be roughly divided into:                water based, the drying mechanism involving absorption, penetration and evaporation;        oil based, the drying involving absorption and penetration;        solvent based, the drying mechanism involving primarily evaporation;        hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification;        UV-curable, in which drying is replaced by polymerization.        
It is also known that dyes used in inks for ink-jet printing must meet different stringent requirements. For example they are required to provide sharp, non-feathered images having good water-fastness, solvent fastness, light-fastness and optical density. Their solubility must be fine-tuned to the vehicle they are dissolved in. Preferably they have high molecular extinction coefficients. In spite of the many dyes that already exist for application in ink-jet inks, there is still a continuous search for novel dyes and especially for dyes with an improved light-fastness and stability towards (singlet)oxygen, ozone and air pollutants such as sulfur oxides (SOx) and nitrogen oxides (NOx).